Drug Metabolizing Enzymes: CYP2D6

• Drug metabolizing enzymes regulate how the body activates, deactivates, degrades, and excretes therapeutics
• Enzymes mainly reside in the liver, the body's main detoxifying organ

Categories of Metabolizing Enzymes

• Phase 1 metabolism enzymes, like the cytochrome P450 family, add or remove hydrogens to change a compound’s charge
• Phase 1 metabolism examples: CYP2C9, CYP2D6, CYP2C19, CYP3A5, DPYD
• Phase 2 metabolism enzymes conjugate chemical groups to change polarity, facilitating its excretion

CYP2D6

• CYP2D6 makes up 4% of liver content
• CYP2D6 metabolizes 25% of clinical drugs
• CYP2D6 is the most studied CYP enzyme in pharmacogenomics (PGx)
• CYP2D6 is not inducible, genotype is key in determining activity
• The Clinical Pharmacogenetics Implementation Consortium (CPIC) recommends those with ultra-rapid CYP2D6 metabolism (UM) avoid codeine

CYP2D6 + Codeine

• Codeine is an opioid analgesic used for mild to moderate pain relief
• Codeine is a prodrug, needing activation by CYP2D6 to form morphine

CYP2D6 Alleles

• Over 100 star (*) alleles exist for CYP2D6
• A combination of CYP2D6 alleles determines a patient's diplotype
• Each allele has an activity score between 0 and 1 based on function
• This activity score assigns a phenotype like ultrarapid or poor metabolizer
• Gene duplications lead to increased activity
• Impaired function alleles = decreased activity
• Null alleles = loss of enzyme activity
• CYP2D6 is metabolized by other enzymes and excreted unchanged
• Approximately 15% of the population is ultra-rapid metabolizers (UM)
  • 61% are normal metabolizers
  • Normal metabolizers' 15% of codeine is metabolized into morphine (analgesic effect)
  • 51.4% are from Argentina
  • 50.4% are from France
  • 24.2% are from UK
• Large variability in alleles that leads to decreased/no function, less in increased
• CYP2D6 duplications=0.3% Sweden, lowest function
• CYP2D6*4: lowest=10% Finland, highest = 33.4% Faroe Islands, 28% Slovakia
• CYP2D6*5: lowest=1% Croatia, 1.3% Sardinia; highest = 6% Norway, 5.9% Denmark
• Those with non-normal alleles are 36.4% likely to have pain (no analgesic effect)
• Codeine is not appropriate for those with CYP2C19 as it has very little influence
• Those carrying allele of normal 15% of worldwide is 36.4%
• Ultra rapid protein as there are normal gene copies but all leads to non functions

CYP2C19 + CLOPIDOGREL

• Clopidogrel is an anti-platelet medication reducing the risk of MI and stroke
• Clopidogrel is an irreversible inhibitor of P2Y12 receptor on platelets
• Clopidogrel blocks adenosine diphosphate (ADP) which mediates platelet activation and aggregation
• Intercellularly, clopidogrel blockade in G-protein signaling which stops activation of glycoprotein and platelet
• Clopidogrel activation is a prodrug
• Prodrug is converted to active metabolite by CYP2C19
• Patients with CYP2C19 poor metabolizer phenotype requires alternative drugs due to risk of clots-CYP2C19*3 w/PM
• Patients with increased CYP2C19 have therapeutic benefit as drug is rapidly converted
• CYP2C19* 17 characterised by c-806 within promoter region (SNP in promoter region residue 1 (start site))

CYP2C19 + CITALOPRAM

• Citalopram is a selective serotonin reuptake inhibitor (SSRI), prescribed antidepressant restoring brain, serotonin level
• CYP2C19 used for Citalopram metabolizing
• Escitalopram (S-enantiomer), citalopram is racemic mixture of S+R enatiomers
• Citalopram is important to inactivation
• Patients w/ a CYP2C19 UM/RM phenotype may have reduced therapeutic benefit as drug rapidly metabolised
• Patients w/ a CYP2C19 IM/PM phenotype may have ADRs as potential for higher plasma concentrations of active drug present
• May consider alternative dose/drug not requiring CYP2C19 for metabolism

Dihydropyrimidine Dehydrogenase (DPYD)

• Dihydropyrimidine dehydrogenase (DPD) is an enzyme (protein) encoded for by the DPYD gene

• This has a key role in first + rate-limiting step of pyramiding metabolism

• C, T, U nucleobases within DNA synthesis

• Key role in the degradation of the anti-metabolite chemo therapies, Fluropyrimidines

• Fluorouracil (5-Fu) -most well known

• Capecitabine, tegafur (prodrugs) – activated to form 5-Fu

• DPD is responsible for 80-90% of the inactive metabolites, a small proportion of the cytotoxic metabolites (cause damage)

• <40% patients on fluoropyrimidine treatment develop serious ADRS

• DPD converts 5-Fu to its inactive metabolite where Toxicity is fatal in 1%

• DPYD gene is present allele in 2-8% patients

• Many variants identified: small number encode DPD with reduced/no activity increasing risk of 5-Fu

• DPYD is different to the SNPs, because the allele determination hasn’t been carried for all of these – only some have been assigned an

• Rs3918290, c.1905+1G>A and DYPD*2A o Mutation one base pair beyond 1905 (1905+1G>A) o Mutation in intron (+1) (just outside coding region, outside exon) – in between 14 + 15 o Results in formation of truncated DPD protein o Everything downstream of 1905+1G>A position 14-15, isn’t produced

• Rs67376798, c.2846A>T, Asp949Val

• There is Decreased DPD function: only a small region of the protein isn’t produced here, leaving some enzyme activity

Fluoracil usage

o Converts 5-FU inactive metabolite o 2-8% inherit DPDP o Many encodes DPD with reduces activity

Dosing of Fluorouracil

• Frequency of various DPYD variants + associated phenotypes varies significantly between different populations
• Prevalence of individuals who are heterozygous for non-functional DPYD variant estimated to be as high as 5-8%, but this varies in different populations
• Majority of evidence linked to the 4 alleles are from white Caucasian populations
• Caucasians, approx 3-5% have partial DPD deficiency
• There are suggestion African/Asian populations at higher risk
• Single test has 4 key variants with:
• C.1905+1G>A (rs3918290) – no function allele - C.2846A>T (rs67376798) – decreased function allele - C.1679T>G (rs55886062) DYPD*13 – no function allele - C.1236G>A (rs56038477) – decreased function allele

TPMT ALLELIC VARIANTS

o It causes AA to be associate W/ a full-DPYD

TPMT

• TPMT = thiopurine S-methyltransferase
• Role in of drug called thiopurines is the Purine anti-metabolites.
• Helps with treat for certain types if Leukaemia.
• Helps the suppressed drug from transplant like Azathiopurine, mercaptopurine, thioguanine
• TPMT catalyses the of thiopurine & is Required for it to go through the body
• Lack is activity builds up to 6-MP in the body & causes Myelosuppression with more
• TPMT prevents conversion into of TGNs

TPMT ALLELIC VARIANTS TPMT PHENOTYPE BASED ON GENOTYPE

TPMT gene is highly polymorphic, with over 40 alleles Most individuals are TPMT that are normal with:

• 90% activity

• TPMT1: is a normal enzyme. TPMT2: c.238G>C, rs1800462 TPMT3A: c.460G>A + c.719A>G, rs1800460 + rs1142345 are have 2 TPMT alleles resulting with high levels of life-threatening suppression TPMT3B: c.460G>A, rs1800460 TPMT*3C: c.719A>G, rs11422345 should be Alternative or has phenotype of dose
• All are in African, Latino, but they are rare in Asian,. Mutations often end up with no protein or too much for the proteins to function